1. Field of the Invention
The present invention relates to an image forming apparatus having an original size detecting apparatus for detecting a size (a length) of an original placed on a document platen.
2. Description of the Related Art
A conventional image forming apparatus such as a copying machine or an image reader is provided with an original size detecting apparatus for automatically detecting a size of an original placed on a document platen. Copy paper of a suitable size is selected according to the detected original size, while the lens is moved according to a copying magnification which depends on the original size detected.
The conventional original size detecting apparatus comprises a photosensor for receiving light reflected from an original and outputted according to intensity of the light, a comparator for comparing the output signal of the photosensor with a specified reference value independent of the original size and outputting a signal indicating presence or absence of the original, whereby the original size is determined when the signal is outputted from the comparator (as indicated in Japanese Patent Publication No. 4974/1985).
However, there are various densities and patterns of originals and intensity of light from an original varies considerably dependent on the density or pattern. For this reason, it is very difficult to set a reference value for determining presence or absence of various originals. In addition, it may happen that the intensity of the reflected light exceeds the reference value in spite of the presence of the original to output a signal indicating absence of an original, and thus it is difficult to detect presence or absence of an original reliably.
Consequently, in the above described conventional original size detecting apparatus, it may happen that the length of an original is determined to be shorter than the actual length, that is, the original size is determined to be smaller than the actual size.
In the above described conventional original size detecting apparatus, the size of the original is determined when a signal indicating absence of an original is outputted from the comparator. Accordingly, the detection operation cannot be started before the exposure lamp for irradiating the original emits a prescribed amount of light stably.
As a result, the detection operation is delayed by a rise time of the exposure lamp causing a decrease in a rising speed (a first copy speed) of the copying apparatus.
Thus, since it is necessary to wait until the amount of light reaches the prescribed value and is applied stably after the turn-on of the light source, the temperature on the document platen increases considerably due to the heat emitted from the light source and particularly the increase of the temperature near the top end of the document platen at the home position (the waiting position) of the light source becomes a problem.
Since the document platen is the portion which the operator touches directly, it is necessary to suppress the temperature to a value lower than a prescribed value for safety. Accordingly, there are difficulties in setting of a capacitor of a cooling fan, setting of a temperature and the like in the image forming apparatus.
In addition, since the irradiating time of the light source is long, there is a large consumption of power as a matter of course.